This invention relates generally to Electric Paper or Gyricons and more particularly concerns a rotating element sheet material in which magnetic fields are used in addition to electric fields for addressing, latching the rotating elements into place once an image has been selected for display, and to provide selected threshold behaviors for individual types of elements. Lee (L. L. Lee, "A Magnetic Particles Display", IEEE Trans. On Elect. Devices, Vol. ED-22, Number 9, September 1975 and L. L. Lee, "Matrix Addressed Magnetic Particles Display", in 1977 Soc. For Information Display International Symposium, Digest of Technical Papers, Boston, April 1977) has described the addressing of a twisting rotating element display in which the rotating elements have a magnetic dipole with magnetic fields. U.S. Pat. No. 3,036,388 by Tate, and issued in May 1962 uses a stylus consisting of a magnetic dipole to address a display consisting of magnetized particles having black and white surfaces corresponding to a given magnetic polarity. More recently, U.S. Pat. No. 5,411,398 by Nakanishi et al. and titled "Magnetic Display System" describes the use of a magnetic dipole to address a display consisting of black ferromagnetic particles and white, non-magnetic particles dispersed in an oil and in turn contained in microcapsules arranged in a layer. Upon application of a magnetic dipole, the black ferromagnetic particles are pushed to the rear of the microcapsules, revealing only the white particles, or pulled to the front of the microcapsules so that mostly only the black ferromagnetic particles can be seen by an observer.
In the above prior art only magnetic fields are used to address twisting or moving magnetic particles and rotating elements. There is no mention or attempt to use electrical fields combined with magnetic fields. U.S. Pat. No. 4,126,854 titled "Twisting Ball Panel Display" issued Nov. 21, 1978, and U.S. Pat. No. 4,143,103 titled "Method Of Making A Twisting Ball Display", issued Mar. 6, 1979, both by Sheridon, describe a twisting rotating element (or "Gyricon") display that comprises bichromal rotating elements contained in liquid-filled spherical cavities and embedded in an elastomer medium. One segment of the bichromal rotating elements has a larger electrical charge in contact with the liquid and in the presence of the electrical field than the other segment. Thus, for a given polarity of applied electrical field, one segment will rotate toward and be visible to an observer of the display. Applying the opposite polarity of electrical field will cause the rotating element to rotate and present the other segment to be seen by the observer.
U.S. Pat. No. 4,143,103 describes the response of the bichromal rotating element to the applied electrical field as a threshold response. That is, as the external field is increased, the bichromal rotating element remains stationary in position, until a threshold voltage is reached, at which time the rotating element starts to rotate from its initial position. The amount of rotation increases with an increasing electrical field until a 180 degree rotation can be achieved. The value of the external field that causes a 180 degree rotation is called the full addressing voltage.
The response pattern of the bichromal rotating element to an external electrical field determines the types of addressing that may be used to create images on the Gyricon display. There are known in the art three types of addressing schemes for displays. The first of these is active matrix addressing, which places the least demands on the properties of the display.
In active matrix addressing a separate addressing electrode is provided for each pixel of the display and each of these electrodes is continuously supplied with an addressing voltage. The complete set of voltages can be changed for each addressing frame. This type of addressing places the least demands on the properties of the display medium, however, active matrix addressing is the most expensive, most complicated and least energy efficient type of addressing.
The second type of addressing scheme is passive matrix addressing. Passive matrix addressing makes use of two sets of electrodes, one on each side of the display medium. Typically, one of these consists of horizontal conductive bars and the other consists of vertical conductive bars. The bars on the front surface or window of the display are necessarily transparent. To address the display medium a voltage is placed on a horizontal conductive bar and a voltage is placed on a vertical conductive bar. The segment of medium located at the intersection of these two bars experiences a voltage equal to the sum of these two voltages. If the voltages are equal, as they usually are, the sections of medium located adjacent to the each of the bars, but not at the intersection of the bars, experience 1/2 the voltage experienced by the section of medium at the bar intersection. Passive addressing is less complicated and more energy efficient because the pixels of the display medium are addressed only for as long as is required to change their optical states. However, the requirements for a medium that can be addressed with a passive matrix display are significantly greater than for the active matrix case. The medium must respond fully to the full addressing voltage but it must not respond to 1/2 the full addressing voltage. This is called a threshold response behavior. The medium must also stay in whichever optical state it has been switched into by the addressing electrodes without the continuous application of voltage, that is it should store the image without power. Passive addressing is the most widely used method of addressing displays and is the lowest cost.
The third type of addressing, and probably the most useful for Electric Paper (paper surrogate) applications, consists of a linear array of addressing electrodes in the form of a bar that can be moved over the surface of the display medium. Typically, the medium is placed over a grounding electrode and is protected from possible mechanical damage from the moving bar by placing a thin window between the bar and the Electric Paper. As the bar is moved over the display medium, it applies voltages to specific pixels of the medium for short periods of time and generates a full image each time the bar is scanned over the surface. In one variation of this method, the addressing bar deposits image-wise charge on the surface of the window.
The requirements imposed on the display medium by this form of addressing then depend on which type of addressing bar is used. If the addressing bar simply exposes the medium to voltages as it passes over the display window, then it is necessary for the display medium to exhibit threshold behavior. Thus the area of the medium directly under the addressing bar electrode must change optical states when exposed to the full addressing voltage, but as the bar moves to the next row of pixels, this same area of medium must not respond to the diminished voltages experienced by the medium from the moving addressing bar. As in passive addressing, this requires that the medium have a sharp threshold response. This addressing bar also requires that the optical state of the medium completely change during the time the addressing bar electrodes move over its vicinity which usually limits the display frame addressing speed. Copending U.S. patent application Ser. No. 09/037,767 by Howard et al. and titled "Charge Retention Islands For Electric Paper And Applications Thereof" also assigned to the same assignee as this application, describes an arrangement of addressing electrodes that greatly reduces the switching speed requirements of the medium due to this effect.
In U.S. patent application Ser. No. 09/037,767 the addressing bar deposits image-wise charge on the surface of the display window. The charge deposition addressing method relaxes the requirements on the display medium. The addressing bar speed over the medium surface is limited only by the rate at which it can deposit image-wise charge, because the medium can respond to the voltage associated with the deposited charge pattern at its own speed. Threshold response behavior is not so important, however the ability to store the image is because it can be expected that the image-wise charge deposited on the window surface will leak off over a short period of time. However, addressing bars that can deposit image-wise charge on the display window tend to be bulky and more expensive than bars that simply impose image-wise voltages directly.
There is a need, therefore, to find other means to control the optical switching characteristics and optical image storage characteristics of Gyricon display media. It is the purpose of this patent application to disclose new and improved means of accomplishing this by the addition of magnetic materials in the composition of the a Gyricon rotating elements and the sheet material and by the use of externally imposed magnetic fields.
Accordingly, it is the primary aim of the invention to provide a means for controlling the optical switching characteristics and the image storage characteristics of gyricon sheets by using magnetic materials and magnetic fields to provide sharp and uniform threshold voltages, provide improved image latching characteristics, and in conjunction with electric fields to provide improved addressing methods.
Further advantages of the invention will become apparent as the following description proceeds.